The invention relates to an electroluminescent device comprising a first electrode, a second electrode and an ionic, organic layer which is in contact with said first electrode, which layer contains a conjugated compound and mobile ions. The invention also relates to a method of manufacturing an electroluminescent device comprising an ionic layer, which layer contains mobile ions.
An electroluminescent (EL) device is a device built up of an electroluminescent layer, which layer emits light when a voltage is applied across electrodes which are in contact with said layer. Such a device can be used, inter alia, as a light source whose light output can be varied in a simple manner by varying the applied voltage. An assembly of independently addressable EL devices, for example in the form of a matrix of light-emitting areas, can be used as a display.
Apart from EL devices based on inorganic materials, such as GaAs, also EL devices based on organic materials are known. Organic EL (oEL) devices on the basis of low-molecular weight materials and on the basis of polymers are known. Known oEL devices are single-layer devices, which means that, apart from the electrodes, the device only comprises the electroluminescent layer, or they are multilayer devices.
The performance of an organic EL device, measured, for example, in terms of the luminance at a specific voltage, depends to a substantial degree on which electrode materials are used. In general, it is assumed that in the case of electrons, the number of electrons injected depends exponentially on the difference between the work function of the electrode and the electron affinity of the organic layer. In the case of holes, the difference between the work function of the electrode and the ionization potential of the organic layer is of corresponding importance. This dependence applies mutatis mutandis also to the EL efficiency, which is defined as the ratio between the number of photons emitted and the number of charge carriers injected, as said EL efficiency is governed by the ratio between the electron current and the hole current. Consequently, it has been found in practice that in the case of, in particular, single-layer devices, the performance necessary for the above-mentioned applications generally can only be achieved if the negative electrode, also referred to as cathode, comprises a metal having a low work function. A low work function is to be understood to mean herein a work function of approximately 3.0 eV or less. A known electrode material, i.e. calcium, meets this criterion. A disadvantage of such metals is that they degrade under the influence of air. Consequently, the service life of EL devices based on such metals is very limited under atmospheric conditions. A known measure to enable metals having a higher work function to be used as the cathode material consists in incorporating additional layers into the device. In general, the manufacture of such multilayer devices is laborious and expensive. Besides, the performance of the device still depends, in principle, on which electrode material is selected: the work function still has to be attuned to the ionization potential and electron affinity of the layers used. The layers and electrodes can only be optimized in conjunction with each other, not separately. Given the multitude of factors which determine the functionality of a layer, such as the layer thickness, the electrical conductivity, the ionization potential, the electron affinity, the band gap and the photophysics, the optimization of a multilayer device is laborious. Consequently, there is a clear need for a simple, single-layer oEL device, which permits electrodes having a high work function to be used without the performance of the device being adversely affected.
Such a device was described recently by Pei et. al., in Science (1995) vol. 269, 1086. In this known device, referred to as xe2x80x9clight-emitting electrochemical cellxe2x80x9d (LEC) by Pei et. al., an electrolyte, for example lithiumtrifluoromethanesulphonate, is added to a layer of a known electroluminescent material, such as a poly(phenylenevinylene), which causes, according to said publication, a p-n junction to be formed in situ by means of electrochemical doping of the EL material. This measure results, inter alia, in that the device emits light already at a voltage which corresponds approximately to the band gap of the EL material and in that EL efficiencies comparable to known polymer-based EL devices (pEL) are achieved while using electrode materials having a high work function such as gold and aluminium.
However, the known LEC has disadvantages. Although the known LEC makes use of electrodes having a high work function, this has no effect on the service life. Said service life is comparable to that of corresponding devices in which a cathode having a low work function is used instead of an electrolyte. As explained hereinabove, the service life of the latter devices under ambient conditions is very limited and definitely insufficient for the intended applications. A further disadvantage is that by means of diffusion the electrolyte can move through every organic layer while preserving its charge neutrality. Consequently, a multilayer construction of the known LEC in which only one layer contains the electrolyte is not feasible. In addition, it is difficult to disperse the electrolyte on a molecular scale in the customary EL materials, which, in general, are non-ionic and predominantly apolar.
It is an object of the invention, inter alia, to provide an oEL device which does not have the above-mentioned drawbacks. The invention specifically aims at an oEL device whose service life under ambient conditions is much better, even without particular protective measures, than that of comparable, known LEC devices, even when the air is saturated with water vapour. Said device should have a good EL efficiency and have a satisfactory light output already at a low voltage. In addition, the EL efficiency of the device should be substantially independent of the work function of the electrodes used, so that it is possible, inter alia, to use a material having a high work function as the cathode material or to use the same material for both the anode and the cathode. The expression xe2x80x9csubstantially independentxe2x80x9d is to be understood to mean herein that the charge injection is no longer determined by the above-mentioned exponential dependence. A further object is to disperse the ions of the electrolyte on a molecular scale. It should be possible to choose the ionic characteristics of a layer substantially independently of the charge-transporting and electroluminescent characteristics of the layer. The expression xe2x80x9ccharge transportxe2x80x9d is to be understood to mean only the transport of electrons and holes necessary for the electroluminescence, not the transport of ions. In the case of a multilayer structure, it should be possible, if necessary, to limit the presence of ions of a specific polarity to one or more layers. It should also be possible to manufacture the single-layer or multilayer EL device in a simple manner. In particular, it should be possible to achieve the intended properties with oEL devices which are exposed to ambient conditions during their manufacture.
These and other objects are achieved by means of an EL device of the type mentioned in the opening paragraph, which is characterized, in accordance with the invention, in that either only negatively charged ions or only positively charged ions are mobile relative to the first electrode. It has been found that the service life, under ambient conditions, of the EL device manufactured in accordance with the invention is much longer than that of comparable, known LEC devices in which both positive and negative mobile ions are used. Said service life is achieved without taking any protective measures. It has even been found that such devices can be operated for days in an atmosphere saturated with water vapour. It has also been found that a service life of several months in combination with good performances can be readily achieved. In a typical example, the EL efficiency was approximately 1.5% and the light output was approximately 500 Cd/m2 at 5 V, while using a gold cathode and an indium tin oxide (ITO) anode.
For the cathode material use can suitably be made of materials having a high work function. In fact, the EL efficiency is substantially independent of the choice of the cathode material. Examples of suitable cathode materials are gold, platinum and other noble metals, aluminium, indium tin oxides.
For the electrode material use is advantageously made of metals which can be provided in liquid form, such as indium. They can be provided in a simple manner and an electrode thus formed proves to be non-porous. The absence of porosity has a favourable effect on the service life.
Said cathode materials can also suitably be used as anode materials. If the EL device has a xe2x80x9csandwichxe2x80x9d structure, it is advantageous to use an electrode material which is transparent to the light to be emitted, such as an indium tin oxide (ITO). The presence of mobile ions compensated by immobile ions creates a xe2x80x9crestoring forcexe2x80x9d if said mobile ions have been moved under the influence of an electric field or diffusion, which restoring force, in the case of multilayer devices as will be described hereinbelow, can be advantageously used. The inventive EL devices can be manufactured in a simple manner, while being exposed to air, by methods which are known per se.
In accordance with the invention, not only mobile ions but also immobile ions are present which serve to compensate the charge of the mobile ions. Charge neutrality is assumed, although it is not a prerequisite for all intended purposes. The mobility of ions depends, inter alia, on the temperature and the matrix in which they are present. For example, the mobility can be increased by gelation by adding a suitable solvent and/or heating. Other important factors are the size of the ion and the strength of the bond between oppositely charged ions. Preferably, a mobile ion is small and soft, and an immobile ion is large. The mobility of a mobile ion should be as high as possible. Dependent upon the applications, a suitable mobility of a mobile ion is 10xe2x88x9214 cm2/Vs or more. The mobility of a suitable immobile ion is approximately 10xe2x88x9219 cm2/Vs or less. Mobile as well as immobile ions should be chemically inert, particularly under the operating conditions of the device.
Suitable mobile anions are ions which are derived from, for example, Bronsted acids, such as halogenides, in particular Ixe2x88x92, tosylates, triflates, carboxylates or Lewis-acid anions, such as BF4xe2x88x92. The mobile anions can be exchanged for others in a simple manner. Suitable mobile cations are, for example, alkaline (earth) metal ions, such as Na+ or K+, or quaternary ammonium compounds, taking the above general guide lines into consideration. In the case of very small cations, such as Li+ or maybe even H+, it is desirable to use an ion-conducting polymer, such as polyethylene oxide.
The ionic layer can only suitably be used in an EL device if a conjugated compound is present which transports the injected charges. If a single-layer device is used, the presence of a conjugated compound having an EL property in the ionic layer will additionally be necessary, which compound is often identical to the charge-transporting compound. By means of mixing or synthesis, the ionogenic compound can be combined with known charge-transporting and EL compounds, such as low-molecular weight fluorescent dyes, in particular coumarines, EL polymers, in particular polyphenylenevinylenes, or high-molecular or low-molecular weight derivatives of phenyl-biphenyl-1,3,4-oxadiazole or triphenylamine dimer or polyvinylcarbazole. It is required, however, that the ionogenic compound leaves the charge-transporting and/or electroluminescent properties of the layer obtained by using the conjugated compound substantially unchanged. This requirement will be met if the ionogenic compound has a much larger band gap and ionization potential and a much smaller electron affinity than the conjugated compound.
The ionic layer can be manufactured by means of methods which are known per se. Layer thicknesses vary typically from 25 to 500 nm, in particular from 50 to 150 nm.
The time-dependence of the current-voltage characteristic (CV) and of the luminance-voltage characteristic (LV) of the EL device in accordance with the invention was found to differ from that of the conventional devices in which no mobile ions are used. In operation, the CV characteristic of the latter devices is initially constant as a function of time, but deteriorates gradually, i.e. as a result of degradation, a constantly increasing voltage is necessary to maintain a constant current. However, the CV and LV characteristics of the device in accordance with the invention improve with time, i.e. the voltage required to obtain a specific current decreases continuously. In other words, at a constant voltage, the current and the luminance increase. Also the EL efficiency of the device improves, values of at least 1.0 to 1.5% being feasible. Only after a long period of time, typically several days to months the performance of the device decreases as a result of degradation. The time interval within which the improvement of the CV characteristic takes place can be shortened by a so-called activating operation. The term xe2x80x9cactivationxe2x80x9d is to be understood to mean that a higher voltage is temporarily applied. This voltage typically is a factor of 2 to 4 higher than the voltage used during the life test. If the device is switched off for a short period of time, typically approximately ten seconds, almost immediately the same characteristic as after activating is obtained. If the device is switched off for a long period of time, for example approximately 10 minutes, the improvement stage has to be covered again. In accordance with the finding that the performance of the device is substantially independent of the electrode materials used, the performance obtained in xe2x80x9creverse biasxe2x80x9d is comparable to that obtained in xe2x80x9cforward biasxe2x80x9d. The stability of the electrode material may differ as a function of the polarity of the applied voltage. It has been found that the activating time depends on the mobility of the ions. Shorter times suffice if the device is heated or if the ionic layer is gelated by means of a solvent. The activating time is also shorter as the layer is thinner.
It has been found that the service life of the device in accordance with the invention can be improved further by using an additional layer. Consequently, a preferred embodiment of the EL device in accordance with the invention is characterized in that said device comprises an additional layer, which layer is situated between the second electrode and the ionic layer and which contains a conjugated compound as well as such a quantity of mobile ions that the overall charge of these mobile ions is substantially compensated by immobile ions of the ionic layer. It is noted that the qualification xe2x80x9cionic layerxe2x80x9d only makes sense in multilayer devices if immobile ions are used, which are substantially absent in the additional layer. Unlike known multilayer devices, the resultant freedom of construction does not have to be sacrificed to the attunement of the electron affinity and ionization potential of the relevant materials to the work function of the electrodes, as electrode-independence is guaranteed substantially by the presence of the ions. Both the additional layer and the ionic layer can be used as an EL and/or charge-transporting layer.
Suitable materials for the additional layer are the known EL and charge-transporting materials, such as a poly(phenylenevinylene). It is alternatively possible to use various additional layers, but this leads to a greater complexity. In a particularly suitable configuration, the second electrode is used as the negative electrode, as in general the injection or charge transport of electrons needs to be improved. A particular, preferred embodiment of the EL device is characterized in accordance with the invention in that the ionic layer and the additional layer have substantially identical fluorescence spectra, ionization potentials and electron affinities. As the difference between the ionic layer and the additional layer consists merely in the presence and absence, respectively, of immobile ions, the conjugated parts can be selected so that the above characteristic is satisfied. This is in contrast to known multilayer devices in which a plurality of layers are used to create differences in ionization potential, electron affinity or fluorescence spectrum. The EL device in accordance with the invention combines the advantages of monolayer and multilayer devices. Such a device can be manufactured in a simple manner by successively providing the two layers or by using an inventive method which will be described in greater detail hereinbelow.
Another preferred embodiment of the EL device in accordance with the invention is characterized in that the immobile ion is formed by a charged substituent which is linked to the conjugated compound by means of a covalent, saturated bond. By combining the ionogenic and conjugated properties in one compound, the necessity of mixing various compounds can be dispensed with. A problem which often occurs during mixing is phase separation. This occurs, in particular, if ionogenic materials have to be mixed with non-ionogenic materials. As regards the intended device, however, it is advantageous to disperse the ions on a molecular scale. The ionogenic property can be introduced synthetically by using a charged group as the substituent of the conjugated compound. By linking the substituent by means of a covalent, saturated bond, the ionogenic property and the conjugated property can be introduced with a minimum of mutual interference. Therefore, suitable compounds can be obtained in a simple manner by combining suitable conjugated and ionogenic compounds.
A particular, preferred embodiment of the EL device in accordance with the invention is characterized in that the immobile ion of the ion layer is formed by a polymer. The use of polymeric materials has advantages. The high-molecular weight ensures that the ionic portions which form part of the polymer are indeed immobile. Further, polymers are, in general, readily processable, amorphous and suitable for producing flexible devices having large surface areas by using simple techniques such as spin coating. Examples of commercially available ionogenic polymers are, for example, polystyrenesulphonate or poly(meth)acrylate. Other polyelectrolytes can readily be obtained synthetically. To ensure dispersion on a molecular scale, it is of course possible again to combine the ionic property and the conjugated property in one compound. In the case of polymers, this is very advantageous. The mixing of two polymers will almost always give rise to phase separation if no special measures, such as the addition of xe2x80x9ccompatibilizersxe2x80x9d, are taken.
A further preferred embodiment of the EL device in accordance with the invention is characterized in that the ionic layer comprises a quaternary amine as the immobile ion. The expression xe2x80x9cquaternary aminesxe2x80x9d is to be understood to mean herein amines which can be obtained from their neutral counterpart by means of an alkylation agent. Consequently, quaternary amines also include quaternized aromatic amines such as the pyridinium compounds. These ions can be provided in the ionic layer, inter alia, by means of an inventive method, referred to as quaternization, which will be explained in greater detail hereinbelow. As a result of the fact that, in this case, the ionic property is not introduced until after the layer has been formed, problems regarding phase separation as a result of the presence of the ions can be precluded. A multilayer device can also be manufactured in this manner.
A particularly suitable embodiment of the EL device in accordance with the invention is characterized in that the ionic layer comprises a conjugated poly(p-phenylenevinylene). Poly-p-phenylenevinylenes are very suitable EL materials. They exhibit a high degree of fluorescence and a satisfactory electroconductivity. The emission spectrum can be varied and readily soluble and processable variants can be obtained by means of substitution, in particular, in positions 2 and 5 of the phenyl ring.
A very suitable, preferred embodiment of the EL device in accordance with the invention is characterized in that the ionic layer comprises a copolymer in accordance with formula (IA), (IB), or (II) 
wherein the degree of polymerization n+m varies from 5 to 1,000,000, R1, R2, R3, R4 are chosen to be equal or unequal to xe2x80x94Xxe2x80x94Rxe2x80x94H or xe2x80x94Rxe2x80x94H, R5 is xe2x80x94Rxe2x80x94K1A1 or xe2x80x94Rxe2x80x94A2K2 and R6 is equal to R5 or to xe2x80x94Xxe2x80x94R5, wherein R is a branched or unbranched C1-C20 alkylene or phenylene-alkylene, X is sulphur or oxygen, K1 is an ammonium group, A1 is selected from the group formed by Ixe2x88x92, Tosxe2x88x92 or other Bronsted-acid anions, A2 is xe2x80x94CO2xe2x88x92 or xe2x80x94SO3xe2x88x92 and K2 is selected from the group formed by NR4+, alkali. These compounds can be synthesized in a simple manner by means of known methods, are soluble and can readily be processed to form amorphous layers in which the ions are dispersed on a molecular scale. Preferably, the fraction m/(n+m) in polymers in accordance with formula (IA/IB) is below 0.15 and above 0.001. Higher values cause the service life to be shortened as a result of an interruption of the conjugation, whereas lower values require an ever longer activating time. In the case of polymers in accordance with formula (II), the fraction m/(n+m) can be varied between 0 and 1, preferably the fraction is greater than 0.001 and smaller than 0.1. The smaller the fraction, the longer the necessary activating time is. At values above 0.1, a substantial improvement is no longer achieved. It has been found that the service life of EL devices prepared by means of polymers (II) is better than that of comparable devices prepared by means of polymers in accordance with formula (IA/IB). It has also been found that, under otherwise equal conditions, the voltage necessary to attain a specific current intensity is lower in devices based on polymers in accordance with formula (II). If polymers in accordance with formula (II) are used, the device can even be operated in air saturated with water vapour for several days.
The presence of non-ionic substituents promotes the solubility. With a view thereto, it is advantageous to choose substituents of unequal length and/or branched substituents. The use of alkylene substituents longer than C20 hardly leads to a further increase in solubility, whereas the quantity of active material is reduced. The solubility is also determined by the nature of the mobile counterion. For example, polymers in which the tosylate ion is used as the counterion can more readily be dissolved in toluene than the same polymer in which iodide is used as the counterion.
A very advantageous embodiment is characterized in accordance with the invention in that the ionic layer comprises a copolymer in accordance with formula (II), wherein the degree of polymerization n+m varies from 5 to 1,000,000, R1 is methoxy, R2 is 3,7-dimethyloctyloxy, R3 is methoxy and R6 is [xe2x80x94CH2CH2N(CH3)3]+Ixe2x88x92.
The invention also relates to a method of manufacturing an EL device. In accordance with this method, a first electrode is provided with an ionic layer on which, subsequently, a second electrode is provided, which method is characterized in accordance with the invention in that the ionic layer comprises a compound which can be alkylated, and, before the second electrode is provided, the ionic layer is exposed to an alkylating agent, so that ions are formed in the regions exposed to said agent. An advantage of this method, referred to as quaternization, is that the ionic property is not introduced until the moment when the morphology of the layer has been fixed, thereby precluding phase separation which could occur as a result of the presence of ions. A further advantage is that a multilayer device can be manufactured from a single-layer device in a simple manner by exposing the layer comprising the compound which can be alkylated, that is the precursor layer, to an alkylating agent for a shorter period of time than would be required for complete alkylation, so that the ionic layer and the additional layer are simultaneously formed from the precursor layer. The transition from the alkylated, ionic layer to the non-ionic, additional layer is given by the diffusion profile of the alkylating agent and will be governed by the selected process conditions. Suitable compounds which can be alkylated are compounds whose alkylated product is stable. Dependent upon the strength of the alkylating agent, it is generally required that the compound to be alkylated comprises a lone pair which is associated with an oxygen atom, sulphur atom or nitrogen atom. Particularly suitable representatives of this class of materials are tertiary amines because, in general, they lead to very stable alkylated compounds. The alkylating agent should be selected so that no undesirable side reactions occur. Suitable alkylating agents for amines are, for example, alkylhalides and alkyltosylates. Particularly suitable alkylating agents are the gaseous methyliodide and alkyltosylates which can be dissolved in customary solvents.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.